The present invention relates to a process for converting methanol and/or dimethyl ether to olefins, particularly light olefins rich in ethylene and propylene.
There is a growing need for light olefins, especially ethylene and propylene, for a variety of uses making it desirable to develop sources of the olefins additional to the conventional source, crude oil. One such additional source is methanol and/or dimethyl ether which can be catalytically converted over certain zeolite catalysts to olefin-containing hydrocarbon mixtures.
For example, U.S. Pat. No. 3,911,041 discloses that methanol can be converted to C2-C4 olefins by contacting the methanol at a temperature of 300-700xc2x0 C. with a crystalline aluminosilicate zeolite catalyst which has a Constraint Index of 1-12, such as ZSM-5, and which contains at least about 0.78% by weight of phosphorus incorporated in the crystal structure of the zeolite.
U.S. Pat. Nos 4,049,573 and 4,088,706 disclose that methanol can be converted to a hydrocarbon mixture rich in C2-C3 olefins and mononuclear aromatics, a, particularly p-xylene, by contacting the methanol at a temperature of 250-700xc2x0 C. and a pressure of 0.2 to 30 atmospheres with a crystalline aluminosilicate zeolite catalyst which has a Constraint Index of 1-12 and which has been modified by the addition of an oxide of boron or magnesium either alone or in combination or in further combination with oxide of phosphorus.
U.S. Pat. No. 4,480,145 discloses that the ethylene yield in the catalytic conversion of methanol over ZSM-5 can be increased by moderating the diffusivity of the zeolite by the use of the large crystal form of the zeolite and by silica xe2x80x9cstuffingxe2x80x9d of the zeolite pores. This patent also discloses that by steaming the zeolite at 180-820xc2x0 C. to reduce its alpha activity to 6-10 the cycle life and methanol conversion activity of the zeolite can be increased. The reaction conditions employed in this patent are a temperature of 260-400xc2x0 C. and a pressure of 1 to 10 atmospheres.
U.S. Pat. No. 4,849,573 teaches that the use of zeolites with a Constraint Index of 1-12 and a silica to alumina molar ratio of 298 to 2000 increases the light olefin yield when methanol is converted to hydrocarbons at a temperature of 350-700xc2x0 C. and a pressure of 1 to 100 atmospheres.
U.S. Pat. No 4,499,314 discloses that the addition of various promoters, including aromatic compounds, such as toluene, accelerate the conversion of methanol to hydrocarbons over zeolites, such as ZSM-5, which have a pore size sufficient to permit sorption and diffusion of the promoter. In particular, the ""314 patent teaches that the increased conversion resulting from the addition of the promoter allows the use of lower severity conditions, particularly lower temperatures, which increase the yield of lower olefins (column 4, lines 17-22). Thus in Example 1 of the patent the addition of toluene as a promoter reduces the temperature required to achieve full methanol conversion from 295xc2x0 C. to 288xc2x0 C. while increasing the ethylene yield from 11 wt % to 18 wt %. In the Examples of the ""314 patent the methanol feedstock is diluted with water and nitrogen such that the methanol partial pressure is less than 2 psia.
The ""314 patent is representative of the current expectation in the art that, in the conversion of methanol over zeolites, such as ZSM-5, the overall selectivity decreases as the partial pressure of methanol is increased and that ethylene selectivity is reduced as the temperature is increased. Thus attempts to maximize ethylene selectivity have focussed on systems operating at low temperature, below 350xc2x0 C., and low methanol partial pressure, below atmospheric. Unfortunately, however, operation at low methanol partial pressure necessarily reduces the perpass ethylene yield and hence the overall efficiency of the process.
Unexpectedly, it has now been found that by using a zeolite having 10-ring intersecting channels, such as ZSM-5, in a catalyst with reduced diffusion characteristics and by operating within narrowly defined, but relatively high temperatures and pressures, methanol can be converted with a high selectivity and yield of C2 to C4 olefins, particularly to ethylene.
The invention resides in a process for converting methanol or dimethyl ether to a product containing C2 to C4 olefins, the process comprising the step of contacting a feed containing methanol or dimethyl ether with a catalyst which comprises a zeolite having 10-ring intersecting channels and which has a Diffusion Parameter for 2,2-dimethylbutane of less than 100 secxe2x88x921 when measured at a temperature of 120xc2x0 C. and a 2,2-dimethylbutane pressure of 60 torr (8 kPa), the contacting step being conducted at a temperature of 370 to 480xc2x0 C., a methanol partial pressure of 30 to 150 psia.
Preferably, the catalyst has a Diffusion Parameter of about 0.1-30 secxe2x88x921.
Preferably, the contacting step is operated so that the methanol conversion per pass is 25-90%.
Preferably, the catalyst contains an oxide of phosphorus.
Preferably, the catalyst contains about 0.05 to about 20 wt %, and more preferably about 1 to about 10 wt %, of said oxide phosphorus on an elemental basis.
Preferably, the zeolite is ZSM-5.